The present invention relates to charge pump circuits, and more particularly to circuitry for converting a unipolar input supply voltage to a bipolar set of output voltages.
Charge pumps have been used for decades to generate larger output voltages from an input supply voltage. Charge pumps employ capacitors and diodes or transistors that regulate current flow to and from the capacitor. A pumping action is produced as charge flows to and from the capacitor. Capacitive coupling can yield a higher output voltage than the input voltage. For example, the output voltage can be double the power-supply input voltage. Pumping can occur over multiple period of time known as phases.
Sometimes both a positive and a negative output voltage are needed. For example, the RS-232 interface has both a positive and a negative power supply. Dual-voltage charge pump circuits are useful for generating these dual voltages for RS232 interfaces.
There are a number of ways to generate two output voltages of opposite polarity. Some charge pump circuits first generate a positive voltage, which is double the input supply voltage, and then invert the voltage polarity to create a negative voltage output. This method of generating dual-voltage outputs is simple and requires very little circuitry, but it has problems. For example, it may take twice as long to first generate the positive and then the negative output voltages. Another problem is that both outputs are dependent upon each other, so electrical noise is passed from one output to the other.
A better method of generating bipolar output voltages from one input supply is to have two charge pump circuits that simultaneously generate two output voltages by controlling separate sets of charge and holding capacitors. In this case, the negative output voltage is generated separately from the positive output voltage although both use the same input voltage source. Because the two charge pump branches are separate and independent, they can drive larger output loads. The two independent branches also generate less cross talk and voltage noise spikes on the outputs, because the positive and negative capacitors are isolated from each other. With the addition of clocks and control circuitry, this charge pump can be programmed to generate single or dual bipolar outputs and pump larger amounts of current into one output over the other output based on changes in load demand (load balancing).
An example of prior art in charge pumps is U.S. Pat. No. 4,999,761. FIG. 1 shows operation of a charge pump with two working states or phases. When charging is required, the charge pump alternates between the first and the second working phases repeatedly. When charging is not required, the charge pump waits at phase 2. The charge pump must traverse these two phases at least two times to get V+ and V− to their required potential. Electronic switches (not shown) perform switching of charge for the different phase configurations of four capacitors 10, 12, 14, 16 (C1, CV+, C2, and CV−, respectively).
Capacitor C110 is fully charged to the potential of Vcc before switching to phase 1. In phase 1, the charge pump connects Vcc to the negative plate of capacitor C110 and at the same time connects the positive plate of capacitor C110 to the positive plate of capacitor CV+ 12. This has the effect of adding the potential on capacitor C110 to the potential on capacitor CV+ 12, doubling the voltage at capacitor CV+ 12 with respect to Vcc. The result is V+=+2Vcc.
The charge pump now switches back to phase 2 where capacitor C110 is again charged to the potential of Vcc. During this phase the switches connect the positive plate of capacitor CV+ 12 to the positive plate of capacitor C214, which transfers the charge from capacitor CV+ 12 to capacitor C214. The result is that the voltage on capacitor C214 is +2Vcc.
When the charge pump switches back to phase 1 the switches connect the positive plate of capacitor C214 to ground and the negative plate of capacitor C214 to the negative plate of capacitor CV− 16. This causes the voltage potential across capacitor C214 to appear as a negative potential with respect to Vcc. At this point the negative potential on capacitor C214 is added to the negative potential on capacitor CV− 16, doubling the voltage at capacitor CV− 16 with respect to Vcc. The result is an output V− equal to −2Vcc. The charge pump must continuously alternate between phase 1 and phase 2 to keep V+ and V− at a potential of 2Vcc.
The flow chart in FIG. 1 illustrates the charge pump switch pattern. During the charge cycle, the first phase 18 is always followed by the second phase 28 until the outputs reach full potential. When both outputs are fully charged, step 30, the charge pump remains in second phase 28. When the load on outputs V+ and/or V− cause degradation in output amplitude, the charge pump starts the charging cycle again.
One of the disadvantages of this charge pump is that all four capacitors must be used in sequence to obtain the correct output voltage. This method wastes energy when only one of the two output voltages requires charging. This charge pump must go through a complete cycle to charge both output voltages even though only one output voltage may have become degraded. Another problem with this type of charge pump is the loss of power from one capacitor to another. This charge pump uses a bucket brigade method of passing charges from capacitor C214 to capacitor CV− 16 to get V− to its charged state. The total voltage passed from V+ to V− is less than V+ because of leakage in capacitor C214. This results is |V−|≦|V+|. This difference can be verified by measuring and comparing the two outputs V+ and V− under load.
The charge pump described as prior art in U.S. Pat. No. 5,306,954 uses a different voltage shifting method to transfer charges to outputs V+ and V−. FIG. 2 shows operation of a four-phase charge pump. Unlike the prior art charge pump described in FIG. 1, the negative bipolar output voltage is not generated from the positive bipolar output voltage. Instead, the negative and positive bipolar output voltages are each generated, in substantially the same manner, using a symmetrical charge transfer technique illustrated by the four phases in FIG. 2. Electronic switches (not shown) create the switching configurations shown in phases 1, 2, 3 and 4 of four capacitors C120, C222, CV+ 24 and CV− 26.
Phase four is the idle state for this charge pump. At this point V+ and V− are at their peak charge and capacitor C120 has a potential equal to Vcc. When V+ or V− needs charging, charge pump circuitry goes through the first, second, third and fourth phases in circular manner. In phase 1 the negative plate of capacitor C120 is connected to the negative plate of capacitor C222. In this configuration, the potential of capacitor C120 is added to capacitor C222 making the voltage on capacitor C222=2Vcc. In phase 2 capacitor C120 is charged again to a potential equal to Vcc. At the same time the charge on capacitor C222 is transferred to capacitor CV− 26. However the charge pump switches have reversed the poles on capacitor C220 so that the positive plate of capacitor C220 is grounded and the negative plate of capacitor C220 is connected to the negative plate of capacitor CV− 26. The result is output V−=−2Vcc. Note that capacitor C222 and capacitor CV+ 24 have swapped positions in the diagram for phases 2, 4 relative to their positions in phases 1,3.
In phase 3 the switches connect the positive plate of capacitor C120 to ground and the negative plate of capacitor C120 to the negative plate of capacitor C222. Because the positive plate of capacitor C222 is connected to Vcc, now capacitor C222=2Vcc. In phase four capacitor C120 is back to its normal configuration, charged to a potential equal to Vcc. At the same time the charge pump switches have connected capacitor CV+ 24 to capacitor C222 allowing capacitor C222 to transfer its charge to capacitor CV+ 24. The result is output V+=+2Vcc. The charge pump repeats these four phases whenever V+ or V− falls below the minimum charge requirement.
FIG. 3 is a flowchart of the four phases illustrated in FIG. 2. Phase 438 is also the idle state. When the charge pump detects that outputs V+ or V− have become degraded, decision 48 is made to begin the charge cycle by entering phase 132. In phase 132, capacitors are charged to a value equal to 2Vcc. In phase 234, the polarity of the charge is reversed and then passed to a holding capacitor leaving output V−=−2Vcc. In phase 336, capacitors are again charged to a value equal to 2Vcc. In phase 438, the charge built up in phase 336, is passed to a holding capacitor leaving output V+=+2Vcc.
This version of charge pump may be considered better than the charge pump described in FIG. 1 because two separate branches are used to obtain the final values of V+ and V−. This method allows the amplitude of |V+| and |V−| to match. However, this method of generating bipolar charges still uses a sequential charge path with a cycle that cannot be changed. If only one of the two outputs, V+ or V−, require charging, the complete charge cycle must be executed, consuming unnecessary power. A better method is to have control circuitry that allows the charge pump to build up charge on V+ or V− separately as load demand changes. Another improvement is to have control circuitry that allows the charge pump to build up unequal voltage potentials on V+ and V−.